Multi-Element Analysis of Cannabis using the Agilent 7800 ICP-MS

Application Note

Food safety

Authors:
Craig Jones and Jenny Nelson
Agilent Technologies, USA

Introduction
In the U.S., marijuana remains a Schedule I controlled substance. Worldwide, marijuana use is permitted for medicinal purposes in countries including Australia, Canada, Croatia, Czech Republic, Macedonia, and Poland. Currently, 29 US states, the District of Columbia, Guam, and Puerto Rico allow for comprehensive public medical marijuana and cannabis programs [1]. This is also the case in some countries including the Netherlands, Spain, South Africa, and Uruguay.

Countries and U.S. states that permit use of medicinal and recreational marijuana require rigorous testing of cannabis and associated products to ensure safety from contaminants, including inorganic impurities such as the toxic elements As, Cd, Pb, and Hg. The analysis of mineral and additional trace elements provides labeling information that is required when these products are used as nutritional supplements. Since contamination can occur during the manufacturing process, analysis is necessary at all stages of production.

Trace element analysis of plant and nutritional supplement materials is well established [2]. Following acidic digestion to break down the primary components of the plant-based samples, ICP-MS is often used for quantitative analysis because of its multi-element capability, high sensitivity, speed, robustness, and wide dynamic range.

In this study, the Agilent 7800 ICP-MS was used to analyze 25 elements in a range of cannabis and cannabis-related products.

Experimental
Instrumentation
A standard Agilent 7800 ICP-MS, which includes Agilent’s proprietary High Matrix Introduction (HMI) system, was used for the analysis. Sampling was performed using an Agilent SPS 4 autosampler. The 7800 ICP-MS was configured with the standard sample introduction system consisting of a Micromist glass concentric nebulizer, quartz spray chamber, and quartz torch with 2.5 mm id injector. The interface consisted of a nickel plated copper sampling cone and a nickel skimmer cone.

Instrument operating conditions are listed in Table 1. The settings for HMI are autotuned as appropriate for the matrix levels of the target sample types. In this case, the HMI dilution factor was 4x. All analytes were acquired in helium (He) collision mode. Using the simple methodology, He mode reliably reduces or eliminates all common polyatomic interferences using kinetic energy discrimination (KED).

Table 1. ICP-MS operating conditions

For comparison purposes, As and Se were also acquired using half-mass tuning, which corrects for overlaps due to doubly charged rare earth elements (REEs). The instrument was automatically tuned for half mass correction in the ICP-MS MassHunter software. The software also collects semi-quantitative or screening data across the entire mass region, referred to as Quick Scan. Quick Scan provides data for elements that may not be present in the calibration standards.

Standard Reference Materials (SRMs)
Various SRMs bought from National Institute of Standards and Technology (NIST) were analyzed in this study to verify the sample preparation digestion process. The SRMs used were NIST 1547 Peach Leaves, NIST 1573a Tomato Leaves, and NIST 1575 Pine Needles. NIST 1640a Natural Water was used to verify the calibration procedure.

Samples
A range of cannabis-based products were analyzed in this study. The samples included cannabis, cannabis tablets, a cannabidiol tincture, chewable sweets, and a hemp-based body cream.

Standard and sample preparation
Calibration standards were prepared using a mix of 1% HNO3 and 0.5% HCl. Na, Mg, K, Ca, and Fe were calibrated from 0.5 to 10 ppm. Hg was calibrated from 0.05 to 2 ppb. All remaining elements were calibrated from 0.5 to 100 ppb.

After weighing the samples (approximately 0.15 g of cannabis plant and between 0.3-0.5 g of cannabis product) into quartz vessels, 4 mL HNO3 and 1 mL HCl were added and the samples were microwave digested using the program given in Table 2. HCl was included to ensure the stability of Ag and Hg in solution. The digested samples were diluted using the same acid mix as the standards. The SRMs were prepared using the same method to verify that the digestion was complete and to confirm the quantitative recovery of the analytes.

Table 2. Parameters for microwave digestion.

Four samples (see Table 6) were prepared in triplicate and fortified with an Agilent Environmental Mix Spike solution (part number 5183-4686) before analysis. The samples, spikes, and SRMs were diluted 5x before analysis to reduce the acid concentration.

Calibration and calibration verification
Representative calibration curves for the critical toxic trace elements As, Cd, Pb, and Hg are shown in Figure 1. All curves show excellent linearity across the calibration range.

Figure 1. Calibration curves for As, Cd, Pb, and Hg.

A summary of the calibration data, including detection limits (DLs) and background equivalent concentrations (BECs) is given in Table 3.

Table 3. Calibration summary data acquired in He mode.

As part of the instrument quality control (QC), NIST 1645a Natural Water was used as an Initial Calibration Verification (ICV) standard. The results given in Table 4 show that the recoveries for all the certified elements present in 1640a were excellent, ranging from 93-104%. A mid-level calibration standard comprising mineral elements at 5 ppm, Hg at 1 ppb and all trace elements at 50 ppb was used as the Continuing Calibration Verification (CCV) solution. The CCV was analyzed six times throughout the run. The mean recoveries and range are also given in Table 4. All CCV recoveries were within ±10% of the expected value.

Table 4. ICV and CCV recovery tests.

Internal Standard Stability

Figure 2. Internal standard signal stability for the sequence of 58 samples analyzed over approximately 4 hours.

Figure 2 shows the ISTD signal stability for the sequence of 58 samples analyzed over ~4 hours. The ISTD recoveries for all samples were well within ±20 % of the value in the initial calibration standard. These ISTD recoveries are comparable to the results obtained routinely using ICP-MS, demonstrating the robustness of the 7800 ICP-MS with HMI.

Results and Discussion
Three SRMs were analyzed to verify the digestion process. The results are given in Table 5. For most elements, the mean 7800 ICP-MS results were in good agreement with the certified concentrations, where certified values are provided. The measured results for As in NIST 1547 and Se in both NIST 1547 and 1573a did not show such good agreement. Some plant materials may contain high levels of rare earth elements (REEs), also known as lanthanides (LA). These elements have low second ionization potentials, so readily form doubly-charged ions (REE++). As the quadrupole mass spectrometer separates ions based on their mass-to-charge ratio (m/z), these doubly-charged ions appear at half their true mass. Doubly-charged ions of the REEs 150Nd, 150Sm, 156Gd, 156Dy, 160Gd, and 160Dy therefore appear at m/z 75, 78 and 80, potentially causing overlaps that can bias the results for As and Se in samples that contain high levels of the REEs. The Agilent 7800 ICP-MS corrects for these interferences using “half mass correction”, which is automatically set up in the ICP-MS MassHunter software. The improvement provided by half-mass correction is illustrated in the corrected results for As and Se shown in the shaded cells in Table 5.

Table 5. Mean concentrations (ppm) of 3 repeat measurements of SRM digests, including certified element concentrations, where appropriate and % Recovery.

Quantitative results
Quantitative results are given in Table 6 for two cannabis-related products, cannabis tablets and a cannabidiol tincture, and two lots of cannabis samples (A and B). Although well below existing regulatory or guideline levels, the concentrations of As (160.0 ppb), Cd (11.33 ppb), Pb (24.00 ppb), and Co (162.1 ppb) were relatively high in cannabis sample A. Pb and Co were also high in cannabis sample B, at 55.40 and 143.4 ppb, respectively.

Spike recoveries
To check the accuracy of the method for actual sample analysis, a spike recovery test was carried out. The four samples were spiked with an Agilent Environmental Mix Spike premixed standard containing multiple elements at 200 ppb, Na, Mg, K, Ca, Fe at 2000 ppb, and Hg at 4 ppb. Using the 7800 ICP-MS direct analysis method, excellent spike recoveries were achieved for most elements in the spiked samples. All recoveries were within ±20% for all elements in the cannabis tablets, a cannabidiol tincture, and two cannabis samples, as shown in Table 6. The spike results for K, Ca, and Mn in the two cannabis samples were invalid as the spike levels were much too low (20 times lower) relative to the levels present in the unspiked samples.

Table 6. Quantitative data for two cannabis-related products and two cannabis samples plus mean spike recovery results. All units ppb apart from major elements, which are reported as ppm.

Conclusions
The analysis of cannabis and associated products is easily performed using the Agilent 7800 ICP-MS. The 7800’s HMI capability enables the routine analysis of samples that contain high and variable matrix levels, while minimizing the need for conventional liquid dilution. By automating dilution in the aerosol phase, manual sample handling steps and the potential for contamination during sample preparation can be reduced, producing more accurate results. Agilent’s ICP-MS MassHunter Quick Scan function provides a complete picture of the elements present in the sample, as data can be reported for elements not included in the calibration standards.

The automated tuning of the ICP-MS for half mass correction allows As and Se to be determined with good accuracy, reducing the impact of doubly charged interference caused by high levels of REEs.

The 7800 ICP-MS is suitable for trace metal screening of medicinal and recreational cannabis, as well as related products. The analysis can be carried out at all stages of production to ensure product quality control and products that are free of toxic metals.

References
1. National Conference of State Legislatures, NCSL, State Medical Marijuana Laws, accessed October 2017, http://www.ncsl.org/research/health/state-medical-marijuana-laws.aspx
2. A. Filipiak-Szok et al. Journal of Trace Elements in Medicine and Biology, 30, 2015, 54–58.
3. P. J. Gray, W. R. Mindak, J. Cheng, Elemental Analysis Manual for Food and Related Products, ICP-MS Determination of Arsenic, Cadmium, Chromium, Lead, Mercury, and Other Elements in Food Using Microwave Assisted Digestion, US Food and Drug Administration publication, 2015.